Battery charger

ABSTRACT

A battery charger outputs a high voltage even when a power supply device having a rechargeable battery is not connected to the battery charger so that the battery charger can supply the high voltage to the power supply device whenever the power supply device is connected to the output terminal of the battery charger. This battery charger has periodic signal producing means for supplying a periodically varying periodic signal to the output terminal by charging and discharging a capacitor connected to the output terminal, and detecting means for checking whether the power supply device is connected to the battery charger or not by checking through a filter whether the periodic signal is present at the output terminal or not. Thus, although the battery charger has no mechanical switch, it can detect the presence of the power supply device.

This application is an International Stage 371 of PCT/JP97/03420 filedSep. 24, 1997.

TECHNICAL FIELD

The present invention relates to a charger for a rechargeable battery.

BACKGROUND ART

In general, a battery charger is so designed that it charges a batteryby supplying it with an electric current while the voltage across thebattery is lower than a predetermined level, and that it thereafterstops charging when the voltage across the battery reaches thatpredetermined level as the result of the charging. FIG. 7 is a blockdiagram illustrating how a conventional battery charger is connected toa power supply device (hereafter referred to as the "battery pack"). InFIG. 7, numeral 60 represents the charger, and numeral 61 represents thebattery pack that is charged thereby. When the voltage across thebattery pack 61 is lower than a predetermined level, the charger 60charges the battery pack 61 by supplying it with a current Ia, andmeanwhile it keeps an LED (light-emitting diode) 64 on to indicate thatcharging is in progress (this LED will hereafter be referred to as the"charge-in-progress LED"). When the voltage across the battery pack 61reaches the above-mentioned predetermined level, the charger 60 stopsthe supply of the current Ia and turns off the charge-in-progress LED64, and in addition it turns on an LED 65 to indicate that charging hasbeen completed (this LED will hereafter be referred to as the"charge-complete LED").

For example, in a case where the battery pack 61 is a power supplydevice that employs lithium-ion cells, the battery pack 61 incorporatesa protection circuit that serves to secure stable operation of thosecells by protecting them against overdischarge and other hazards. As aresult, when overdischarge or the like is detected, this protectioncircuit inhibits the discharging of the battery pack 61 from within it.In this state, where both charging and discharging are inhibited, thecharging of the battery pack 61 cannot be restarted without firstcanceling this inhibiting state. For this reason, the charger 60 is sodesigned that, even when it is not connected to the battery pack 61, itoutputs a voltage almost as high as the voltage across the battery pack61 in its fully charged state, and on the other hand the battery pack 61is so designed that it cancels the above-mentioned inhibiting state bydetecting a feeble current that flows into it when it is connected tothe charger 60 and receives therefrom the above-mentioned high voltage.

At this time, however, precisely because the charger 60 is designed tooutput a high voltage on its current-supplying side, the charger 60,even when it is not connected to the battery pack 61, keeps thecharge-complete LED 65 on, falsely indicating that charging has beencompleted. To overcome this inconvenience, the conventional charger 60is, as shown in FIG. 7, fitted with a mechanical switch 63 for checkingwhether the battery pack 61 is present or not, so that, when it is notconnected to the battery pack 61, it can keep both thecharge-in-progress LED 64 and the charge-complete LED 65 off. Here, themechanical switch 63 is a switch that has a contact that is mechanicallyopened and closed depending on whether the battery pack 61 is present ornot.

Having such a mechanical switch 63, the conventional charger 60 is proneto malfunction due to imperfect mechanical contact in the mechanicalswitch 63, and thus it does not offer satisfactory safety. In addition,the use of the mechanical switch 63 increases the cost of the charger60.

DISCLOSURE OF THE INVENTION

According to the present invention, a battery charger that outputs ahigh voltage even when a power supply device having a rechargeablebattery is not connected to the battery charger so that the batterycharger can supply the high voltage to the power supply device wheneverthe power supply device is connected to an output terminal of thebattery charger is provided with periodic signal producing means forsupplying a periodically varying periodic signal to the output terminal,and detecting means for checking whether the power supply device isconnected to the battery charger or not by checking whether the periodicsignal is present at the output terminal or not.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a block diagram of a battery charger embodying the presentinvention;

FIG. 2 is a detailed circuit diagram of the pulse current producingcircuit used in the battery charger of the invention;

FIG. 3 is a circuit diagram of the high-pass filter used in the batterycharger of the invention;

FIG. 4 is a diagram showing the waveform of the voltage appearing at theoutput terminal of the battery charger of the invention when no batterypack is connected thereto;

FIG. 5 is a diagram showing the output characteristic of theconstant-voltage/constant-current circuit used in the battery charger ofthe invention;

FIG. 6 is a circuit diagram of the charge control FET and the protectioncircuit incorporated in the power supply device; and

FIG. 7 is a block diagram showing how a conventional battery charger isconnected to a battery pack.

BEST MODE FOR CARRYING OUT THE INVENTION

Hereinafter, an embodiment of the present invention will be describedwith reference to the drawings. FIG. 1 is a block diagram of theprincipal portion of a battery charger embodying the invention. Thisbattery charger is formed as an IC (integrated circuit) 1. The IC 1 hasa terminal (OUT) to which a battery pack 2 is connected when it ischarged. The battery pack 2 is connected in parallel with an outputcapacitor 3, which is provided outside the IC 1.

The IC 1 also has a terminal (RLED) to which a charge-in-progress LED 4is connected, and a terminal (GLED) to which a charge-complete LED 5 isconnected. As will be described later, when the battery pack 2 is notconnected to the terminal (OUT), the LEDs 4 and 5 are both kept off. Onthe other hand, when the battery pack 2 is connected, only thecharge-in-progress LED 4 is kept on when charging is in progress, andonly the charge-complete LED 5 is kept on after the voltage of thebattery pack 2 reaches a predetermined level.

The IC 1 further has a terminal (Vcc) that is connected to a powersource voltage 6 on which the IC 1 operates, a terminal (GND) that isconnected to a ground level voltage, and a terminal (CT) that isconnected to a capacitor 7. The capacitance of this capacitor 7determines the oscillation frequency f of the oscillator (OSC) 10incorporated in the IC 1. An oscillation signal having the frequency fis fed to a control circuit 11 included in a pulse current producingcircuit 30.

The control circuit 11 turns on and off current source circuits 12 and13 by feeding them with a signal that is synchronous with theoscillation frequency f When the current source circuit 12 is off, thecurrent source circuit 13 is on, allowing a current I1 to flow; when thecurrent source circuit 12 is on, the current source circuit 13 is off,allowing a current I2 to flow.

As a result, when the battery pack 2 is not connected to the outputterminal (OUT), the output capacitor 3, which is connected to the outputterminal (OUT), is charged or discharged, and this causes the voltage atthe output terminal (OUT) to vary periodically, as shown in FIG. 4,between a full-charge voltage (Ve) and a predetermined voltage (Vt)lower than that. Here, the voltage (Vt) is set, for example, to 99% ofthe full-charge voltage (Ve), although the proportion is not restrictedto any specific value. Depending on the characteristics of the actualcircuit and other factors, the waveform may not always be triangular asshown in FIG. 4, but may be square or sawtooth-shaped, for example. Thethreshold level (Vref) will be described later.

By contrast, when the battery pack 2 is connected to the output terminal(OUT), since the battery pack 2 has a low impedance, the voltage at theoutput terminal (OUT) is not affected by the current I1 or I2, and thusit is kept equal to the voltage of the battery pack 2. A pulse-drivestarting comparator 14 checks whether the voltage of the battery pack 2is equal to or higher than the voltage (Vt). If the voltage of thebattery pack 2 is lower than the voltage (Vt), the control circuit 11 isturned off so that the pulse current producing circuit 30 will bedeactivated. On the other hand, when the voltage of the battery pack 2is equal to or higher than the voltage (Vt), the control circuit 11 isturned on so that the pulse current producing circuit 30 will beactivated.

This causes the battery pack 2 to be charged by the pulse currentproducing circuit 30 and thus causes its voltage to increase gradually.However, since the pulse current producing circuit 30 is, as shown inFIG. 2, so designed that it operates only when the voltage at the outputterminal (OUT) is in the range from the voltage (Vt) to the full-chargevoltage (Ve), it does not occur that the battery pack 2 is chargedbeyond its full-charge voltage (Ve). Specifically, the comparator 14compares the voltage at the output terminal (OUT) with the voltage (Vt),another comparator 18 compares the voltage at the output terminal (OUT)with the full-charge voltage (Ve), and the outputs of these comparators14 and 18 are fed through an AND gate 27 to the control circuit 11 forits turning-on/off operation. Thus, the pulse current producing circuit30 operates only when the voltage at the output terminal (OUT) is in therange from the voltage (Vt) to the full-charge voltage (Ve). Note that,in FIG. 2, such elements as are found also in FIG. 1 are identified withthe same reference numerals and symbols.

In FIG. 1, when the battery pack 2 is not connected to the terminal(OUT), an overdischarged battery recovering circuit 16 keeps the voltageat the terminal (OUT) equal to the voltage (Vt). This allows the batterypack 2, even when it is in the state in which charging is inhibited dueto overdischarge, to receive a high voltage from the overdischargedbattery recovering circuit 16 and thereby cancel the inhibiting state assoon as it is connected to the terminal (OUT). A pulse detectioncomparator 17 compares the voltage at the output terminal (OUT) with athreshold level (Vref), which is set, as shown in FIG. 4, to a voltagebetween the full-charge voltage (Ve) and the voltage (Vt).

When the battery pack 2 is not connected to the terminal (OUT), thepulse detection comparator 17 outputs a signal that oscillates with aperiod f in synchronism with a signal fed from the pulse currentproducing circuit 30. On the other hand, when the battery pack 2 isconnected, the pulse detection comparator 17 outputs a fixed high or lowlevel depending on the voltage of the battery pack 2. The signaloutputted from the comparator 17 is passed through a high-pass filter 19to extract only the alternating-current components thereof.

The signal flowing out of the high-pass filter 19 is compared with areference voltage (Vs) by a comparator 20. The high-pass filter 19 iscomposed of, as shown in FIG. 3, a capacitor 19a and a resistor 19b, forexample. Note that, in FIG. 3, such elements as are found also in FIG. 1are identified with the same reference numerals and symbols.

Reverting to FIG. 1, when the battery pack 2 is connected to theterminal (OUT), the pulse detection comparator 17 outputs either a fixedhigh level or a fixed low level, and thus causes the comparator 20 tooutput a low level. On the other hand, when the battery pack 2 is notconnected to the terminal (OUT), the comparator 20 outputs a signal thatoscillates regularly with a period f. The output of the comparator 20 isfed to a delay circuit 22. The delay circuit 22 is composed of aswitching transistor 23, a capacitor 24, and a current source circuit25. The voltage across the capacitor 24 is compared with a referencevoltage (Vu) by a power-supply-device detecting comparator 26.

When the battery pack 2 is not connected to the terminal (OUT), thesignal flowing out of the high-pass filter 19 oscillates, and thiscauses the transistor 23 to be alternately turned on and off, and thuscauses the capacitor 24 to be alternately charged and discharged. As aresult, the voltage across the capacitor 24 becomes sufficiently low,and the power-supply-device detecting comparator 26 outputs a low level.

On the other hand, when the battery pack 2 is connected to the terminal(OUT), the transistor 23 is kept off, and therefore the capacitor 24 ischarged by the current source circuit 25. When the voltage across thecapacitor 24 exceeds the reference voltage (Vu), the power-supply-devicedetecting comparator 26 outputs a high level.

In this way, by the use of the signal outputted from thepower-supply-device detecting comparator 26, it is possible to checkwhether the battery pack 2 is connected or not. The comparator 26 mayalso be so configured that, using completely inverted logic, it outputsa high level when the battery pack 2 is not connected and a low levelwhen battery pack 2 is connected.

When the comparator 26 feeds a high level to an LED driving circuit 28,the LED driving circuit 28 turns on either the charge-in-progress LED 4or the charge-complete LED 5, and thereby indicates that the batterypack 2 is connected to the terminal (OUT). By contrast, when thecomparator 26 outputs a low level, the LED driving circuit 28 keeps bothLEDs 4 and 5 off, and thereby indicates that the battery pack 2 is notconnected to the terminal (OUT).

The signal outputted from the power-supply-device detecting comparator26 is fed also to a constant-voltage/constant-current circuit 29. Whenthe battery pack 2 is not connected to the terminal (OUT), theconstant-voltage/constant-current circuit 29 is turned off. By contrast,when the battery pack 2 is connected, theconstant-voltage/constant-current circuit 29 is turned on, and exhibitsan output characteristic as shown in FIG. 5. As shown in FIG. 5, theconstant-voltage/constant-current circuit 29 is so configured that itdoes not perform charging when the voltage of the battery pack 2 islower than a charge-inhibition level (Vj). The charge-inhibition level(Vj) is determined in accordance with the type of the battery used.

More specifically, owing to the above-mentioned output characteristic,in the voltage range 55 from around the charge-inhibition level (Vj) toaround the voltage (Vt), constant-current charging is performed; as thevoltage of the battery approaches the voltage (Vt) with the progress ofthe charging, the charging current is so reduced that the voltage iskept almost constant. Meanwhile, when the charge-completion point 54 isreached during the charging, the constant-voltage/constant-currentcircuit 29 (see FIG. 1) feeds a signal to the LED driving circuit 28 torequest it to change the indication from "charge-in-progress" to"charge-complete". Thus, the LED driving circuit 28 turns off thecharge-in-progress LED 4 and instead turns on the charge-complete LED 5to indicate that the charging has been completed.

When the charging further progresses and the voltage of the batteryreaches the voltage (Vt), the constant-voltage/constant-current circuit29 is deactivated. Then, the pulse-drive starting comparator 14activates the pulse current producing circuit 30 to start the supply ofa periodic signal to the battery pack 2. The pulse current producingcircuit 30 outputs the periodic signal in such a way that the batterypack 2 is charged gradually by the periodic signal. Accordingly, in theinterval 52 shown in FIG. 5, the voltage of the battery gradually rises.When the voltage of the battery reaches the full-charge voltage (Ve),the pulse current producing circuit 30 (see FIG. 2) is deactivated, andthus the charging is completed.

In FIG. 5, the broken line 57 indicates the load characteristic of thefeeble current I that is fed from the overdischarged battery recoveringcircuit 16. This line 57 shows that the overdischarged batteryrecovering circuit 16 supplies a current to the battery back 2 even inthe interval 55, but that this current is far smaller than the chargingcurrent supplied from the constant-voltage/constant-current circuit 29.This line 57 also shows that, when the battery pack 2 is not connectedto the terminal (OUT), the overdischarged battery recovering circuit 16charges the output capacitor 3 in order to raise the voltage at theterminal (OUT) up to the voltage (Vt) and thereby activate the pulsecurrent producing circuit 30.

As described above, the IC 1 can not only charge the battery pack 2, butalso check whether the battery pack 2 is connected or not without usinga mechanical switch and thereby turn on and off the LEDs 4 and 5properly. Moreover, free from malfunction due to imperfect contact suchas experienced with a mechanical switch, the battery charger of theembodiment provides higher safety. Furthermore, the elimination of themechanical switch leads to the reduction of the cost of the batterycharger.

Next, a description will be given as to the protection and othercircuits incorporated into the battery pack 2. FIG. 6 shows an exampleof the internal circuit of the battery pack 2. The battery pack 2essentially consists of a battery proper 32 such as a lithium-ion cell,a protection circuit 31, and an n-channel FET (field-effect transistor)33 for discharge control. When the battery 32 is in the process ofdischarging, the FET 33 is normally kept on, and the dischargedelectricity is extracted via a positive (+) terminal 35 and a negative(-) terminal 36.

The voltage of the battery 32 is fed through a resistor R1 to a terminal(V) of the protection circuit 31. When the voltage of the battery 32reaches the discharge-inhibition level (Vg) as the result of thedischarging of the battery 32, a comparator 37 shifts its output from ahigh level to a low level. This level shift is fed through an OR gate 38to a terminal (FE), and is outputted therefrom to turn off the FET 33.This inhibits the discharging of the battery 32 and thereby prevents thedeterioration of the characteristics thereof.

However, in this state, in which the FET 33 is off, it is not possibleto start the charging of the battery 32 immediately. First of all, it isnecessary to apply a voltage close to the full-charge voltage of thebattery 32 between the positive (+) and negative (-) terminals 35 and36. This causes a feeble current I to flow through the body diode 34 ofthe FET 33, and thus causes a voltage drop there. This voltage drop isfed through a resistor R2 to a monitor terminal (MO) of the protectioncircuit 31, where a comparator 39 compares the drop with a voltage (Vf)and outputs, in this case, a high level. This high level is fed throughthe OR gate 38 to the terminal (FE), and is outputted therefrom to turnon the FET 33. Now it is possible to start the charging of the battery32. Note that the body diode 34 is merely a parasitic diode, asexplicitly illustrated, that exists in the FET 33. The protectioncircuit 31 also has a terminal (GND) through which it receives areference level.

It is to be understood that the embodiment described specifically aboveis merely one example of how the present invention can be applied. Forexample, it is also possible to use, in place of the high-pass filter 1,any circuit that can detect alternating-current components in a signal;it is also possible to incorporate the oscillation capacitor 7 into theIC 1.

INDUSTRIAL APPLICABILITY

As described heretofore, according to the present invention, in abattery charger, a periodic signal is produced and fed to the outputterminal so that, by checking whether the periodic signal is present atthe output terminal or not, it is possible to check whether a batterypack is connected or not. This makes it possible to detect the presenceof the battery pack without the use of a mechanical switch dedicated tosuch detection. The elimination of a mechanical switch leads not only tothe elimination of malfunction due to imperfect contact or the like andthus to increased safety, but also to the reduction of the cost. Inparticular, in a battery charger for a battery pack, such as employs alithium-ion cell, that is fitted with a protection circuit forprotection against overdischarge and other hazards and that keeps itsoutput voltage almost as high as the full-charge voltage even when nobattery pack is connected to its output terminal, it is possible todetect the presence of the battery pack even when the battery charger iskeeping its output at such a high voltage.

We claim:
 1. A battery charger that outputs a predetermined voltage evenwhen a power supply device having a rechargeable battery is notconnected to said battery charger so that said battery charger cansupply said predetermined voltage to said power supply device wheneversaid power supply device is connected to an output terminal of saidbattery charger, comprising:periodic signal producing means forsupplying a periodically varying periodic signal to said outputterminal; and detecting means for checking whether said power supplydevice is connected to said battery charger or not by checking through afilter whether said periodic signal is present at said output terminalor not.
 2. A battery charger as claimed in claim 1, furthercomprising:charge completion indicating means; and indication disablingmeans for disabling said charge completion indicating means when saiddetecting means yields an output that indicates that said power supplydevice is not connected to said battery charger.
 3. A battery charger asclaimed in claim 1,wherein said battery is a lithium-ion cell.
 4. Abattery charger as claimed in any one of claims 1 to 3,wherein saidperiodic signal producing means produces said periodic signal when avoltage at said output terminal is in a predetermined range close to avoltage of said power supply device in its fully charged state.
 5. Abattery charger comprising:an output terminal through which a current issupplied to a power supply device having a rechargeable battery;periodic signal producing means for supplying a periodically varyingperiodic signal to said output terminal; a first capacitor connected tosaid output terminal in such a way that said first capacitor isconnected in parallel with said power supply device; a detectioncomparator for checking whether said periodic signal is present at saidoutput terminal; a high-pass filter connected to an output side of saiddetection comparator; a switching device that is turned on and off inaccordance with an output of said high-pass filter; a second capacitorthat is charged and discharged according as said switching device isturned on and off; and power-supply-device detecting comparator forcomparing a voltage across said second capacitor with a referencevoltage.
 6. A battery charger as claimed in claim 5, furthercomprising:an overdischarged battery recovering circuit for keeping avoltage at said output terminal close to a voltage of said power supplydevice in its fully charged state when said power supply device is notconnected to said output terminal.